Any ophthalmic lens intended to be held in a frame involves a prescription. This ophthalmic prescription can include a positive or negative power prescription as well as an astigmatism prescription. These prescriptions correspond to corrections enabling the wearer of the lenses to correct defects of his vision. A lens is fitted in the frame in accordance with the prescription and the position of the wearer's eyes relative to the frame.
For presbyopic wearers, the value of the power correction is different for far vision and near vision, due to the difficulties of accommodation in near vision. The prescription thus comprises a far-vision power value and an addition (or power progression) representing the power increment between far vision and near vision; this comes down to a far-vision power prescription and a near-vision power prescription. Lenses suitable for presbyopic wearers are progressive multifocal lenses; these lenses are described for example in FR-A-2 699 294, U.S. Pat. No. 5,270,745 or U.S. Pat. No. 5,272,495, FR-A-2 683 642; FR-A-2 699 294 or also FR-A-2 704 327.
Progressive multifocal ophthalmic lenses include a far-vision zone, a near-vision zone, and intermediate-vision zone, a principal progression meridian crossing these three zones. They are generally determined by optimization, based on a certain number of constraints imposed on the different characteristics of the lens. Most marketed lenses are all-purpose lenses, in that they are adapted to the different needs of the wearers at the time.
A progressive multifocal lens can be defined by geometric characteristics on at least one of its aspherical surfaces. In order to characterize an aspherical surface, the parameters constituted by the minimum and maximum curvatures at each point are conventionally used, or more commonly their half-sum and their difference. This half-sum and this difference multiplied by a factor n−1, n being the refractive index of the lens material, are called mean sphere and cylinder.
Moreover, a progressive multifocal lens can also be defined by optical characteristics taking into account the situation of the wearer of the lenses. In fact, the laws of the optics of ray tracings provide that optical defects appear when the rays deviate from the central axis of any lens. Conventionally, the aberrations known as power defects and astigmatism defects are considered. These optical aberrations can be generically called obliquity defects of rays.
Obliquity defects of rays have already been clearly identified in the prior art and improvements have been proposed. For example, the document WO-A-98 12590 describes a method for determination by optimization of a set of progressive multifocal ophthalmic lenses. This document proposes defining the set of lenses by considering the optical characteristics of the lenses and in particular the wearer power and oblique astigmatism, under wearing conditions. The lens is optimized by ray tracing, using an ergorama associating a target object point with each direction of viewing under wearing conditions.
EP-A-0 990 939 also proposes to determine a lens by optimization taking into account the optical characteristics instead of the surface characteristics of the lens. For this purpose the characteristics of an average wearer are considered, in particular as regards the position of the lens in front of the wearer's eye in terms of curving contour, pantoscopic angle and lens-eye distance.
It has been found that each wearer has different lens-eye behaviour. Recently therefore it has been sought to personalize progressive ophthalmic lenses in order to best satisfy the needs of each wearer.
For example, it is proposed, in particular by ZEISS and RODENSTOCK under the references Zeiss Individual® and Impression ILT® respectively, to take account, for the definition of progressive lenses, of the real position of the lens in front of the wearer's eye. For this purpose, measurements of the position of the lens in the frame chosen by the wearer are carried out. The measurement of the position of the lens relative to the wearer's eye is initially difficult to carry out with precision. Then, the optimization is carried out for a measured position of the lens in front of the wearer's eye; it turns out that the position of the frame varies over time and cannot be considered to be constant for a given wearer. Because of these two factors, the consideration of the position of the lens does not seem to provide the wearer an additional comfort compared to solutions which consider only the mean position of the lens.
The applicant markets, under the trade mark VARILUX IPSEO® a range of progressive lenses, which are defined as a function of the wearer's head-eye behaviour. This definition is based on the fact that any wearer, in order to look at different points at a given height in the object space, can move either his head, or his eyes and that the viewing strategy of a wearer is based on a combination of head and eye movements. The wearer's viewing strategy influences the perceived width of the fields on the lens. Thus, the more the wearer's lateral vision strategy involves a movement of the head, the narrower is the zone of the lens scanned by the wearer's vision. If the wearer moved only his head in order to look at different points at a given height of the object space, his vision would still pass through the same point of the lens. The product VARILUX IPSEO® therefore proposes different lenses, for the same ametropia-addition pair, as a function of the wearer's lateral vision strategy.
Moreover, the documents U.S. Pat. No. 6,637,880 and U.S. Pat. No. 6,871,955 describe ophthalmic lenses optimized by taking into account the real position of the centre of rotation of the wearer's eye, referenced CRE. The lens-CRE distance is defined as the sum of the distance lens-cornea (referenced VC) and cornea-CRE (referenced CR). The value VC is a function of the wearing conditions and the value CR is linked to the measurement of the axial length of the eye. The axial length can be measured by the optician or by the optometrist for each individual and the position of the CRE is deduced from this by a rule of three. The axial length of the eye can be measured for example with the device marketed under the trade mark IOLMaster® by ZEISS.
For the optimization of progressive ophthalmic lenses, documents U.S. Pat. No. 6,637,880 and U.S. Pat. No. 6,871,955 propose taking into account the fact that the CRE is situated at different distances when the wearer is looking in far vision, in near vision or in any other point of the lens and to integrate it in the optimization. For example, it is indicated that a change of the lens-CRE distance has an impact on the lateral shift in near vision; the optical design is therefore calculated as a function of this value. It is also indicated that the lens-CRE distance has an impact on the power required in far vision; the asphericity of the lens is therefore modified as a function of this value.
The applicant has also developed a device for measuring the position of the centre of rotation of the eye of a given individual, which is the subject of the French Patent Application filed by the applicant under the title Method and device for the determination of the centre of rotation of an eye on 8 Apr. 2005 under number FR 05 50902 (now published under number FR-A-2 884 130).
The measurements of the axial length of the eye or of the centre of rotation of the eye are carried out by the optician or the optometrist; they are difficult to carry out and the apparatus is relatively expensive. In addition, these measurements are not used to determine the distributions of power and resulting astigmatism defects on the optimized lens.
Tests carried out in the applicant's laboratories have shown that the axial length of the eye influences the wearer's perception of the fields and gradients. A need still exists therefore for a lens which better satisfies the specific needs of each individual wearer.